This invention relates to capacitive accelerometers and, in particular, to single-side microelectromechanical capacitive accelerometers and methods of making same.
The constant drive toward small size, lightweight, low cost and low-power sensing systems in all application domains has made high sensitivity and precision microaccelerometers increasingly needed.
These accelerometers are used in numerous applications, such as inertial navigation and guidance, space microgravity measurements, seismology and platform stabilization. Also, as they become manufacturable at low cost with small size, they attain a large potential consumer market in their application as a GPS-aid to obtain position information when the GPS receivers lose their line-of-sight with their satellites.
High precision accelerometers are typically operated closed-loop to satisfy dynamic range, linearity and bandwidth requirements, in addition to high sensitivity and low-noise floor.
Capacitive microaccelerometers are more suitable since they provide stable DC-characteristics and high bandwidth. Such accelerometers may be fabricated by surface micromachining or bulk micromachining. The surface micromachined devices are fabricated on a single silicon wafer. However, they generally have low sensitivity and large noise floor, and thus cannot satisfy the requirements of many precision applications.
Some high resolution accelerometers are bulk micromachined and use multiple wafer bonding as part of their manufacturing process. This wafer bonding is a complex fabrication step, and hence results in lower yield and higher cost. Also, forming damping holes in the thick bonded wafers is difficult, and thus special packaging at a specified ambient pressure is typically needed to control the device damping factor. Finally, due to wafer bonding, these devices show higher temperature sensitivity and larger drift especially if glass wafers are used.
The above-noted patent application entitled xe2x80x9cMicroelectromechanical Capacitive Accelerometer And Method Of Making Samexe2x80x9d utilizes a single wafer fabrication technology with damping holes. However, fabrication of the accelerometer requires double side processing and lead transfer from both sides of the wafer. As shown in FIG. 1, the accelerometer, generally indicated at 10, includes a proof mass 12 suspended by compliant beams 14 between two fixed and rigid electrodes 16. In the presence of an external acceleration, the proof mass 12 moves from its center position and thus CS1 and CS2 change in opposite directions. The proof mass 12 is rebalanced to its center position by applying an electrostatic force to either the top electrode 16 or the bottom electrode 16.
U.S. Pat. No. 5,345,824 discusses a monolithic capacitive accelerometer with its signal conditioning circuit fabricated using polysilicon proof mass and surface micromachining.
U.S. Pat. No. 5,404,749 discusses a boron-doped silicon accelerometer sensing element suspended between two conductive layers deposited on two supporting dielectric layers.
U.S. Pat. No. 5,445,006 discusses a self-testable microaccelerometer with a capacitive element for applying a test signal and piezoresistive sense elements.
U.S. Pat. No. 5,461,917 discusses a silicon accelerometer made of three silicon plates.
U.S. Pat. No. 5,503,285 discusses a method for forming an electrostatically force rebalanced capacitive silicon accelerometer. The method uses oxygen implantation of the proof mass to form a buried oxide layer and bonding of two complementary proof mass layers together. The implanted oxide layer is removed after bonding to form an air gap and release the proof mass.
U.S. Pat. No. 5,535,626 discusses a capacitive microsensor formed of three silicon layers bonded together. There is glass layer used between each two bonded silicon pairs.
U.S. Pat. No. 5,540,095 discusses a monolithic capacitive accelerometer integrated with its signal conditioning circuitry. The sensor comprises two differential sense capacitors.
U.S. Pat. No. 5,559,290 discusses a capacitive accelerometer formed of three silicon plates, attached together using a thermal oxide interface.
U.S. Pat. No. 5,563,343 discusses a lateral accelerometer fabricated of a single crystal silicon wafer.
U.S. Pat. No. 5,605,598 discloses a monolithic micromechanical vibrating beam accelerometer having a trimmable resonant frequency and method of making same.
U.S. Pat. Nos. 5,594,171 and 5,830,777 disclose capacitance-type acceleration sensors and methods for manufacturing the sensors. The sensors include a mass portion having a plurality of movable electrodes. The sensors also include a plurality of stationary electrodes. The sensors are manufactured on a single-side of a substrate.
U.S. Pat. No. 5,665,915 discloses a semiconductor capacitive acceleration sensor. The construction of the sensor includes a base substrate having a first electrode attached to the top of the substrate. The sensor also includes a second electrode positioned between the substrate and the first electrode. The first electrode is a stationary electrode and the second electrode is a movable electrode.
U.S. Pat. No. 5,719,336 discloses a capacitive acceleration sensor having a first fixed electrode, a second fixed electrode, a first movable electrode, and a second movable electrode. The stationary electrodes are positioned in a configuration surrounding the movable electrodes.
U.S. Pat. Nos. 5,392,651; 5,427,975; 5,561,248; 5,616,844; and 5,719,069 disclose various configurations of microminiature accelerometers having both stationary and movable electrodes, wherein the electrodes are arranged in various configurations.
The paper entitled xe2x80x9cAdvanced Micromachined Condenser Hydrophonexe2x80x9d by J. Bernstein et al, Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., June, 1994, discloses a small micromechanical hydrophone having capacitor detection. The hydrophone includes a fluid-filled variable capacitor fabricated on a monolithic silicon chip.
The paper entitled xe2x80x9cLow-Noise MEMS Vibration Sensor for Geophysical Applicationsxe2x80x9d by J. Bernstein et al., Digest of Hilton-Head Solid State Sensor and Actuator Workshop, pp. 55-58, June, 1998, discloses an accelerometer having trenches etched in its proof mass to reduce damping and noise floor.
The paper entitled xe2x80x9cHigh Density Vertical Comb Array Microactuators Fabricated Using a Novel Bulk/Poly-Silicon Trench Refill Technologyxe2x80x9d, by A. Selvakumar et al., Hilton Head, S.C., June 1994, discloses a fabrication technology which combines bulk and surface micromachining techniques. Trenches are etched and then completely refilled.
Numerous U.S. patents disclose electroplated microsensors such as U.S. Pat. Nos. 5,216,490; 5,595,940; 5,573,679; and 4,598,585.
Numerous U.S. patents disclose accelerometers such as U.S. Pat. Nos. 4,483,194 and 4,922,756.
U.S. Pat. No. 5,146,435 discloses an acoustic transducer including a perforated plate, a movable capacitor plate and a spring mechanism, all of which form a uniform monolithic structure from a silicon wafer.
An object of the present invention is to provide a single-side, microelectromechanical capacitive accelerometer including a fixed electrode suspended between a proof mass and a moving electrode to provide differential capacitance measurement and force-rebalancing.
Another object of the present invention is to provide a single-side, microelectromechanical capacitive accelerometer formed from a single wafer with a proof mass having a thickness substantially equal to the thickness of the wafer, controllable/small damping and large capacitance variation.
Yet another object of the present invention is to provide a single-side, microelectromechanical capacitive accelerometer wherein packaging and potential integration of the device with its CMOS interface circuitry is simplified since all interconnect leads are on one side of the wafer.
In carrying out the above objects and other objects of the present invention, a microelectromechanical capacitive accelerometer manufactured on a single-side of a semiconductor wafer is provided. The accelerometer includes a fixed electrode, a movable proof mass having a top surface and a movable electrode. The movable electrode is attached to the top surface of the proof mass to move therewith. The fixed electrode is suspended between the proof mass and the movable electrode.
The accelerometer has an input axis and, preferably, both of the electrodes are sufficiently stiff to electrostatically force-balance proof mass displacement due to acceleration along the input axis without substantial bending of the electrodes along the input axis.
Each of the electrodes includes a planar layer which is relatively thin along the input axis and, preferably, at least one of the planar layers is dimensioned and is formed of a material so that its electrode is stiff along the input axis.
Each of the electrodes may include an electroplated planar layer.
The accelerometer may include upper and lower beams for suspending the proof mass in spaced relationship from the fixed electrode.
Preferably, the fixed electrode includes a plurality of stiffeners extending from its planar layer along the input axis to stiffen the fixed electrode. The stiffeners extend toward the proof mass from their planar layer. The proof mass includes a plurality of cavities on its top surface. The stiffeners are received within the cavities. The stiffeners and the proof mass have a substantially uniform, narrow air gap therebetween.
The planar layer and the stiffeners are preferably formed of different forms of the same material such as a semiconductor material like silicon.
Also, preferably, each of the planar layers of the electrodes has a plurality of damping holes formed completely therethrough.
The proof mass is typically formed from a single silicon wafer having a predetermined thickness and wherein the thickness of the proof mass is substantially equal to the predetermined thickness. At least one of the planar layers and the proof mass are, preferably, formed of different forms of semiconductor material.
Further in carrying out the above objects and other objects of the present invention, a single-side, microelectromechanical capacitive accelerometer having an input axis is provided. The accelerometer includes first and second spaced conductive electrodes. Each of the conductive electrodes includes a planar layer which is relatively thin along the input axis, but is stiff to resist bending movement along the input axis. The accelerometer also includes a proof mass which is thicker than either of the planar layers by at least one order of magnitude along the input axis. The accelerometer further includes a first support structure for supporting the proof mass in spaced relationship from the first conductive electrode, and a second support structure for supporting the second conductive electrode on the proof mass. The second conductive electrode moves with but is electrically isolated from the proof mass. The second conductive electrode and the proof mass move together in opposite directions relative to the first conductive electrode. The conductive electrodes and the proof mass form a pair of substantially uniform, narrow air gaps on opposite sides of the first conductive electrode. The conductive electrodes and the proof mass form a pair of acceleration-sensitive capacitors.
Preferably, both of the conductive electrodes are sufficiently stiff to electrostatically force-balance proof-mass displacement due to acceleration along the input axis without substantial bending of the conductive electrodes along the input axis.
At least one of the planar layers may be dimensioned and is formed of a material so that its conductive electrode is stiff along the input axis.
At least one of the planar layers may be an electroplated planar layer.
The first conductive electrode preferably includes a plurality of stiffeners extending from its planar layer along the input axis to stiffen the first conductive electrode. The stiffeners extend towards the proof mass which includes a plurality of cavities which receive the stiffeners. The stiffeners and the proof mass have one of the substantially uniform, narrow air gaps therebetween. The planar layer of the first conductive electrode and the stiffeners are formed of different forms of the same material. Preferably, the material is a semiconductor material such as silicon.
The first conductive electrode preferably comprises a plurality of co-planar, electrically isolated conductive electrodes.
Preferably, the proof mass is formed from a single silicon wafer having a predetermined thickness. The thickness of the proof mass is substantially equal to the predetermined thickness.
Also, preferably, the planar layer of at least one of the conductive electrodes has a plurality of damping holes formed completely therethrough.
The first support structure preferably includes a plurality of beams for suspending the proof mass at upper and lower portions of the proof mass.
Yet still further in carrying out the above objects and other objects of the present invention in a method for making a high-sensitivity, microelectromechanical capacitive accelerometer including a proof mass having a thickness along an input axis of the accelerometer and first and second conductive electrode from a single semiconductor wafer having a predetermined thickness, an improvement is provided. The improvement includes the steps of depositing first and second planar layers on a single-side of the wafer. The planar layers are relatively thin along the input axis. The method also includes the step of stiffening the first and second planar layers to form the first and second conductive electrodes, respectively, which are stiff so as to resist bending movement along the input axis. The method then includes the step of forming substantially uniform first and second narrow gaps between the first conductive electrode and the proof mass and between the second conductive electrode and the first conductive electrode, respectively. The thickness of the proof mass is at least one order of magnitude greater than either the thickness of the first planar layer or the thickness of the second planar layer.
Preferably, the thickness of the proof mass is substantially equal to the predetermined thickness of the wafer and the semiconductor wafer is a silicon wafer.
The step of stiffening may include the step of forming a stiffening metallic layer on at least one of the planar layers.
The step of stiffening may include the step of forming stiffening ribs on at least one of the planar layers.
The step of forming the stiffening ribs, preferably, includes the steps of forming trenches in the proof mass and refilling the trenches with a sacrificial layer having a substantially uniform thickness and an electrode material. The step of forming the substantially uniform, first narrow air gap then includes the step of removing the sacrificial layer.
The step of stiffening may include the step of electroplating the first and second planar layers.
The method may also include the step of forming a plurality of beams for supporting the proof mass at upper and lower portions of the proof mass.
Several significant innovative features of the accelerometer structure and its manufacturing technique include: 1) forming both fixed and moving sense/feedback electrodes with embedded damping holes using stiffened deposited polysilicon layers or electroplated metal layers; 2) forming electrically-isolated stand offs on a top surface of a thick proof mass for the moving sense/feedback electrode; 3) forming uniform narrow gaps over a large area between the two electrodes and between the fixed electrode and the proof mass by etching sacrificial layers; and 4) using a single silicon wafer for manufacturing on a single-side thereof the accelerometer without any need for wafer bonding.
The sensor typically is operated in a closed-loop mode. Preferably, a switched-capacitor, sigma-delta modulator circuit is utilized to force-rebalance the proof mass and provide direct digital output for the accelerometer.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.